This invention relates to electrical bushings of the type consisting of a porcelain housing concentrically arranged about a continuous metal feedthrough conductor. The porcelain envelope prevents electrical discharges from occurring between the electrical conductor and the transformer casing when the bushing is mounted into the casing.
In order to prevent the bushing from becoming overheated when large currents are being transmitted to and from the transformer, the porcelain housing generally contains a filling of insulating oil as a liquid coolant. The natural convection of the oil within the sealed bushing unit keeps the bushing from overheating. The oil efficiently carries heat from the hotter regions of the bushing to the cooler regions in a continuous convection process.
When very high voltages and currents are to be carried by oil filled bushings, it is necessary to provide a conductor of substantial size to ensure sufficient cooling or to connect a plurality of bushings in an electrical parallel relationship. In some applications an aperture is provided within the bottom portion of the bushing that is in contact with the transformer oil. The oil from the transformer itself is then caused to circulate up through the bushing in order to insulate and cool the bushing conductor. U.S. Pat. No. 3,626,079 issued on Dec. 7, 1971 discloses an electrical bushing using the transformer cooling oil to cool the bushing conductor.
An alternative method for cooling high power bushings is to provide a separate heat exchanger and pump arrangement for circulating oil through the bushing for cooling purposes. U.S. Pat. No. 1,983,371 issued Dec. 4, 1934 discloses a high temperature bushing having a separate heat exchanger for cooling the bushing oil.
With the recent government restrictions on the nature of transformer cooling oil, the price of acceptable oil can be prohibitive in large size power transformers. The use of a plurality of large oil filled bushings further adds to the total quantity of oil and further increases the overall economics of the system. The purpose of this invention is to provide an electrical bushing for high power transformers with a minimum of transformer oil and with optimum bushing cooling efficiency.